1. Field of the Invention
The present invention relates to an uplink optical transmission system included in a CATV system for performing bi-directional communications by using a bi-directional transmission line which is formed by connecting an optical fiber and a coaxial transmission line.
2. Description of the Background Art
Conventionally, it has been known that a network of a CATV (Cable Television) system can be constructed by using a bi-directional transmission line called an HFC (Hybrid Fiber Coax) which is formed by connecting an optical fiber and a coaxial transmission line. The HFC is disclosed in, for example, Japanese Patent Laid-Open Publication No. 11-284999 (1999-284999). For the purpose of achieving bi-directional communications between a CATV station and a plurality of subscriber devices, the CATV system using an HFC has a network structure in which each subscriber device belongs to any one of subscriber networks, and such subscriber networks are connected to the CATV station. The CATV station includes a CATV station apparatus, while each subscriber n network includes a node. The CATV station apparatus and each node are connected via an optical fiber as a trunk line, while the node and each subscriber device are connected via a coaxial transmission line as a distribution line. Particularly in recent years, the number of CATV providers using a CATV system with an HFC to provide Internet services is increasing.
FIG. 16 is an illustration showing the configuration of a conventional uplink optical transmission system included in a CATV system using an HFC. The system illustrated in FIG. 16 includes a CATV station apparatus 100, a plurality of optical fibers 200, a plurality of nodes 300, and a plurality of subscriber networks 400. The system allows access from subscriber terminals (not shown) connected to the subscriber networks 400. Each node 300 converts an electrical signal transmitted from a subscriber terminal via the corresponding subscriber network 400 to an optical signal for transmission via the corresponding optical fiber 200 to the CATV station apparatus 100.
The CATV station apparatus 100 includes a plurality of optical receiving sections 110, a plurality of signal separating sections 120, a coupling section 140, a demodulating section 150, and a cable modem termination system 160. The optical receiving sections 110 and the signal separating sections 120 correspond to the respective subscriber networks 400. Each of the optical receiving sections 110 receives an optical signal transmitted from the corresponding subscriber network 400 via the corresponding optical fiber 200 for conversion to an electrical signal. Each of the signal separating sections 120 separates the electrical signal output from the corresponding optical receiving section 110 into a data communication signal and other signals. The coupling section 140 couples a plurality of data communication signals output from the signal separating sections 120. The demodulating section 150 demodulates a signal output from the coupling section 140. The demodulated signal output from the demodulating section 150 is supplied as an uplink signal to the cable modem termination system 160.
In the above-described conventional CATV uplink optical transmission system, optical signals transmitted from the nodes 300 via the optical fibers 200 are individually converted to electrical signals, and these electrical signals are then coupled and demodulated. With such configuration, the subscriber networks 400 can share the single demodulating section 150. Therefore, by using the existing CATV system, Internet services can be provided at low cost.
However, the conventional CATV uplink optical transmission system has some drawbacks. For example, in the CATV station apparatus included in the above conventional system, a plurality of signals received by the plurality of optical receiving sections are coupled by the coupling section, and are then demodulated by the demodulating section. Therefore, the demodulating section may receive an aggregate of noise, for example, occurring at the subscriber networks. Such noise is called ingress noise, causing a problem in configuring the CATV uplink optical transmission system.
Moreover, particularly in recent years, for the purpose of expanding a service area covered by a single optical fiber, a optical transmitting section of higher output power than ever is used in some cases. It is generally known, however, that an optical signal of large power incident to an optical fiber causes a non-linear phenomenon typified by stimulated Brillouin scattering. Such a non-linear phenomenon greatly degrades noise characteristics of the optical transmission system.
In a downlink line of a CATV system, however, the above-described non-linear phenomenon typified by stimulated Brillouin scattering does not occur because of the following reason. In the downlink line, video signals always flow, and therefore optical signals passing through the optical fiber are always modulated. This makes the optical spectrum spread over a wide frequency band even with higher output power of the optical transmitting section, thereby not increasing the peak power of the spectrum by much. Also, in a general communications system, such as a point-to-point system, the above non-linear phenomenon, even if it occurs while no signal to be transmitted exists to increase noise, does not particularly pose a problem because no signal to be affected by the noise exists at all.
In an uplink line of the CATV system, on the other hand, a non-linear phenomenon does occur, which would not occur when a conventional optical transmitting section of low output power is used or when modulated light is output due to the existence of an uplink signal. In the uplink line, whilst monitor signals for monitoring the transmission line and, particularly in recent years, uplink signals from a cable modem are also transmitted, there yet occurs a period during which no signal exists (burst period). During such a period, unmodulated light is output from the optical transmitting section, causing the above non-linear phenomenon in the optical fiber. As described above, in the CATV uplink optical transmission system, a plurality of optical signals transmitted via a plurality of optical transmission systems are individually converted to electrical signals, and these electrical signals are then coupled. For this reason, noise occurring in one optical transmission system can considerably affect communications in other optical transmission systems.